Grain size effect on magnetic and phase transition features in one-dimensional S = 1/2 Heisenberg spin chain molecular crystals

In this study, we prepared sub-micron crystals of one-dimensional (1D) spin-Peierls-type compounds, 1-(4′-R-benzyl)pyridinium-d₅ bis(maleonitriledithiolato)nickelate (R = Br or Cl), using a facile method, namely, an acetonitrile solution of each compound was quickly mixed with excess water (an insoluble solvent). This facile method of preparation gave a uniform dispersion of sub-micron crystals with a typical dimension of <1.0 μm. We observed that the reduction in grain size affected the magnetic and phase transition features. With respect to the bulk crystal samples, the powder X-ray diffraction peaks are broadened, the transition temperature (T_C) is up-shifted with ΔT_C ≈ 1.2 K for R = Br vs. 1.0 K for R = Cl, and the changes of enthalpy and entropy of the phase transition are significantly decreased for the sub-micron crystal samples; in addition, reducing the crystal grain size leads to the onset of a strongly Curie-Weiss-type paramagnetic background and significant temperature-independent paramagnetism.